This invention relates to a water heater, particularly to a water heater for a vehicle having a burner unit and a combustion chamber surrounded by a heat exchanger so as to define an annular outflow space therebetween through which combustion gases flow to an exhaust outlet after emerging from the combustion chamber and being deflected in an opposite direction.
Water heaters of the initially mentioned type are used, preferably, as heaters or auxiliary heaters in motor vehicles. In the case of the heater transfer systems of conventional water heaters, not only the combustion tube forming the interior sheathing consists of metal, but also the exterior shell is made of steel or cast metal. As a result, the water heater, as a whole, becomes heavy, and the use of materials, such as steel or cast metal, also places limits on the shapes used for these parts because shapes have to be selected that can be manufactured cost-effectively, for example, by means of die casting. In addition, cast metals, such as steel, have a thermal conductivity that results in heat being radiated which leads to radiation losses reducing the efficiency of the whole water heater. Especially disadvantageous is the fact that the exterior shell of the water heater, which is also the exterior part of the housing, heats up during operation of the water heater and can, therefore, be installed only at correspondingly suitable locations of the motor vehicle. In most cases, surface treatments of the exterior shell are also required in order to render it passive.
On the basis of German Auslegeschrift No. 28 47 525 or European Pat. No. 0 029 859, it is known in the case of a heat exchanger for evaporators, especially for air conditioning systems, to make a connecting box for a heat exchanger for evaporators of a plastic material which takes into account the special requirements in the case of evaporators, namely a distribution of the liquid coolant or refrigerating agent that is as uniform as possible, by means of a corresponding design.
German Auslegeschrift No. 27 28 827 discloses a heat exchanger wherein a firm connection exists between the water pipes made of metal and connecting boxes made of plastic.
From German Gebrauchsmuster No. 80 09 473, a heater for motor vehicles is known having a bank of pipes delimited by two pipe plates to which a water box is connected in each case, the water boxes being made of one-piece plastic preforms.
On the basis of DE-OS No. 20 13 754, a heater is known, especially an air heater for motor vehicles, where a ring-shaped space exists, into one end of which fresh air is fed in and at an opposite closed end, the fresh air is deflected into another ring-shaped space. In this case, the casing delimiting the first ring-shaped space is formed of two shells made of plastic. Since in the case of this embodiment, the fresh air is guided along the interior surface of the halves of the shells that are made of plastic, no difficulties have to be expected in connection with excessive heating. In particular, no gas-tight, pressure-resistance connection of metal parts and plastic parts, that is also resistant to changes in temperature, is required in such an air heater because the heater does not contain any liquid, such as water, that may result in leakages, in the heater, which may make the heater useless when such liquids are used and may possibly result in damage to the motor vehicle.
Despite the above-mentioned knowledge, it was, therefore, assumed in the field of water heater construction, that the parts that come in contact with water used a heat transfer medium must be made of a generally resistant and corrosion-proof steel or cast metal in order to form the ring-shaped space through which the heat transfer medium flows and to insure that it is closed off in a gas-tight and pressure-resistant fashion. The initially described difficulties were, therefore, accepted in order to ensure a long-term reliable operation of such a water heater.
In addition, it was usually customary, as shown in U.S. Pat. No. 3,779,229, to discharge the hot combustion gases, after heat has been exchanged with the heat transfer medium, via an exhaust outlet extending through the ring-shaped space and through which the heat transfer medium flows, so that localized hot spots occur in the area of the exhaust outlet due to the temperature of the gases being exhausted. A similar result occurs where the end of the space through which the heat transfer medium flows is closed, in the area of the exhaust outlet, by a cast piece through which the combustion gases flow as in the heater shown in U.S. Pat. No. 4,395,225. Such passages will only be durable and tight if the materials that come into contact with one another have similar characteristics, so that one is forced to make the casing parts delimiting the ring-shaped space for the liquid heat transfer medium of the same material as the exhaust outlet or of a material with similar thermal properties. In the case of a water heater, it should especially be taken into account that the water, as the heat transfer medium, must be directed through the ring-shaped space of the heat transfer device below a certain working pressure so that all existing connections must be pressure-resistant and gas-tight.
Thus, the invention has a primary object of overcoming the above-described difficulties by providing a water heater of the above-mentioned type which will be suitable for large-quantity production, be of compact design, have a reduced dead weight and, especially, maintain the space through which the heat transfer medium flows reliably closed off in a gas-tight and pressure-resistant manner.
According to the invention, this is achieved by means of the heat exchanger having an outer casing made of plastic and an inner casing of metal between which the heat transfer medium flows, the inner and outer casings being connected in a manner which is gas-tight and pressure-resistant despite temperature changes.
Surprisingly, it was found that the outer casing, as a delimiting wall of the ring-shaped space through which the heat transfer medium flows, which, at the same time, forms the wall of the exterior housing of the water heater, can be manufactured of plastic so that the dead weight of such a heater can be reduced considerably. At the same time, the manufacturing of the outer casing of plastic also opens up the possibility that the shape of the outer casing can be designed more freely because construction requirements do not have to be met that exist in the case of a cast metal workpiece. In particular, it was, however, also surprisingly found that the connection of an outer casing made of plastic and an inner casing made of metal can be made to be gas-tight, pressure-resistant and stable with respect to temperature changes so that, relative to its tightness, no disadvantages have to be expected in comparison to a unit having an outer casing that is made of metal.
In the case of the invention, the use of a plastic outer casing for the heat exchanger is made possible, especially, because of the fact that the outlet for the hot combustion gases is located in the proximity of the burner unit and does not extend through the ring-shaped space between the inner and outer casings and through which water flows as the heat transfer medium, nor do the gases exit through a part closing an end of said ring-shaped space. In particular, the connection between the outer casing made of plastic and the casing made of metal is spaced from the exhaust outlet for the combustion gases so that a problem will not result at the connection as a result of an excessive development of heat, temperatures in the area of the connection being in the range that the water has as the heat transfer medium. That is, the temperature of the water as the heat transfer medium is significantly lower than the temperature of the combustion gases at the exhaust outlet so that the connecting point, in the case of the water according to the invention, is arranged in a so-called cool zone.
In addition to a considerable reduction in weight as compared to the use of an outer casing that is made of steel, the invention, by making the outer casing of plastic, also permits the outer casing to remain cooler, i.e. that lower heat losses occur through radiation and, thus, a higher efficiency is achieved for the water heater. Since the outer casing, in the case of the water heater according to the invention, is made of plastic and, therefore, heats up less than an outer casing made of metal, the development of heat by the water heater does not have to be taken into account when the water heater is installed. Also, no surface treatments are required for an outer casing that is made of plastic so that the water heater is extremely suitable for mass production because additional treatment steps, previously required, do not have to take place. Preferably, the plastic of the outer casing is a thermosetting plastic. Without surface treatments, this outer casing will then be saltwater-proof and corrosion-proof.
According to the invention, an improved efficiency of the water heater is, therefore, obtained in a constructively simple manner, and the dead weight of the water heater is, at the same time significantly reduced which is an advantage, especially, from the standpoint of installation in motor vehicles.
In accordance with preferred embodiments, the connection of the outer casing made of plastic with the inner casing made of metal are gas-tight, pressure-resistant and stable with respect to temperature changes, and the connections can in each case be adapted to the required operational conditions of the water heater. These connections are simple to manufacture as well as to assemble. In the case of the glued connection according to one embodiment, for example, drying adhesive varnishes are used which do not etch the parts to be connected with one another, and since they set because of a chemical reaction, they reliably connect the outer casing made of plastic with the inner casing made of metal.
In order to ensure a reliable sealing of the connection, either, for example, an O-ring seal may be used or a sealing paste which is applied at the contact surfaces in the area of the connection between metal and plastic casings.
The use of an outer casing of plastic leads to another surprising simplification with respect to manufacturing in that baffle plates may be developed in the shape of water-guiding webs on the inside surface of the outer plastic casing made and need not be shaped of metal at the inner casing, as previously, for reasons of casting practice. Such a development results in greater freedom of design inherent in producing plastic parts as compared to, for example, cast metal parts.
When, as according to the invention, the outer casing is made of plastic, the water inlet and outlet pieces are either molded directly onto the plastic outer casing, without resulting in manufacturing difficulties, or the inlet and outlet pieces may be manufactured separately, for example, of metal parts joined to the plastic casing during molding thereof or may be connected thereto in a manner that is similar to the connection of the plastic outer casing made of plastic to the metal inner casing.
Even if the water inlet and outlet pieces are unitarily formed of plastic with the outer casing, they can still be connected with connecting pieces made of metal, to which the connecting pipes will then be connected. In regard to the installation of this type of water heater, advantages are achieved, because, then, the connection from the hose connection and the connecting piece to the inlet and outlet piece is developed in the conventional way while pairing metal and hose-connecting materials.
Finally, the use of an outer casing made of plastic, according to the invention, also makes it possible to have additional parts molded onto its outer surface which may, for example, be used for receiving a cable harness, a cable duct, or holding means for the heater or supporting means for the heater. In addition, more openings may also be provided in the plastic outer casing without difficulties, for example, for associated parts, such as thermostats, etc.
As demonstrated, the forming of the outer casing of plastic according to the invention permits not only a considerable reduction of the dead weight of the water heater, but also other unexpected and surprising advantages that are not directly connected to the weight reduction.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.